1. Field of the Invention
The present invention relates generally to epoxy-type impregnating compounds, which are useful for electrical potting or encapsulation, plastic tooling, and fiber-reinforced composites. More particularly, the present invention relates to epoxy impregnants having a low exotherm and a low temperature cure.
2. Description of Related Art
In the fabrication of electronic devices and circuits, the electronic devices are protected from the adverse effects of the environment by filling the empty spaces between components on the device with a polymeric material which encapsulates these components. If the electronic device is used outdoors, such as in automobiles, aircraft, marine, or building applications, the potting material or encapsulant must provide protection over extreme outdoor temperature and humidity conditions.
Present high voltage potting and encapsulation, such as for high voltage power supplies, involves the use of epoxy materials having curing temperatures on the order of 160.degree. F. (71.degree. C.). Yet, while stress analysis has indicated that a lower cure temperature would reduce critical stress levels, such materials are not currently available. An example of such a system is a formulation of EPON 815 (a mixture of diglycidyl ether of bisphenol A and a monoepoxide diluent of butyl glycidyl ether, available from Shell Chemical Company) and menthane diamine. This formulation is safe from an exotherm standpoint, develops suitable mechanical and electrical properties, and functions well as an encapsulant; however, it must be cured at temperatures of 160.degree. F. (71.degree. C.) and higher, which results in the creation of a residual stress condition upon cooling to room temperature.
Most epoxy compounds, which are capable of being fully cured at low cure temperatures (defined herein as less than about 120.degree. F., or about 49.degree. C.), also have a propensity towards generation of high exotherm temperatures due to polymerization reactions. Such release of heat can become an unacceptably dangerous situation from the standpoints of worker safety and fire prevention. Generally, for epoxy resin/curative mixtures, the peak exotherm reaches ever higher temperatures as the volume of compound is increased. Temperatures exceeding the decomposition temperature of some epoxy compounds are reached in large masses with catastrophic results.
A need exists for a family of epoxy formulations that cure at low temperatures and generate modest, acceptable exotherms.